Radio communication systems are known in the art. Simulcast radio communication systems, are known to include a plurality of transmitter locations that substantially simultaneously transmit the same modulated information (e.g., digitized audio, data) on the same radio frequency (RF) carrier. This allows the information to be received by a widely spaced group of users. Simulcast system designers are faced with numerous problems that, while not unique to simulcast operation, require more complex solutions than their conventional communication system counterparts. In particular, the fact that users can receive transmitted signals from multiple locations that have propagated different distances produces a notable problem of simulcast operation--i.e., so-called multipath delay spread, which is defined as multiple signals having substantially the same RF carrier frequency and substantially the same modulation being received with a significant time differential or relative delay. This delay limits maximum site separation.
Moreover, requirements for spectrum efficiency tend to reduce the inherent tolerance for multipath delay, making this issue play an even more important role in the design scheme for a modern simulcast system. Spectrum efficient systems are defined as those having a low channel bandwidth-to-modulation symbol rate ratio. One example of a simulcast design challenge arises when attempting to merge acceptable simulcast properties with an otherwise incompatible communications environment, such as the so-called APCO-Project 25 system environment. This system is designed for use as the highly spectrum efficient digital communication approach for the next generation of public safety two way radios. This system has a narrow channelization (12.5 kHz) with proportionally high modulation symbol rate (4.8 kHz)--i.e., channel spacing/symbol rate ratio of 2.6. Such systems typically can tolerate multipath delay spreads of only about 40 microseconds.
Unfortunately for system designers, this limited delay immunity is problematic, as it constrains maximum site separation to approximately eight miles. Typical users, however require a design approach that obtains improved spectrum efficiency, while operating near the current system maximum 20 mile site separation (i.e., equivalent to approximately 100 microsecond delay). In the past, these problems have been resolved using a unique frequency modulation to produce a simulcast signal that is incompatible with conventional APCO Project 25 (hereafter, "APCO-25") receivers, as next described.
FIG. 1 shows a simplified block diagram of a prior art data transmitter 100. Information to be presently transmitted is represented by a data stream 102, which is inputted to a multi-level encoder 103 followed by a low pass filter 104. The filtered components are then digitally modulated using an FM modulator 106, as later described. The modulated waveform is then amplified using an amplifier stage 108 before being transmitted over the air, via a conventional antenna 110. This constant amplitude, frequency modulation approach is standard for today's APCO-25 transmitters, as it allows the use of more efficient and lower cost non-linear power amplifiers 108.
FIG. 2A shows a polar constellation diagram 200 that typifies the FM modulation scheme illustrated in FIG. 1. A constant envelope phasor (i.e., constant amplitude independent of phase magnitude) 203 describes the magnitude circle of the modulation phasor. In this representation, the rate of phase angle change over time defines each of four data levels in the constellation. For example, the time required for the phasor 203 to make the transition from a phase angle 205 (i.e., at position 201) to the phase angle shown at position 209 (i.e., denoted by dashed phasor 207) defines a particular data value. It should be noted that this modulation scheme requires a relatively constant rate of transition between constellation rotation speeds to limit transmitter power spectrum.
FIG. 2B shows a frequency domain transmitter power spectrum characteristic curve 230 for the constant envelope frequency modulation scheme (e.g., as defined in the TIA APCO-25 Air Interface) employed by the transmitter shown in FIG. 1. The power spectrum curve 230 illustrates the relationship between the peak power density (PPD) and frequency (f). Regulatory agencies (e.g., Federal Communications Commission, FCC) generally control this power spectrum to prevent interference with adjacent channel users by a maximum limit curve 240 (referred to herein as "mask 240"). Expressed mathematically, the sideband spectrum specification is given by:
f.sub.d &lt;2.5 kHz; when Att=0 dB; PA1 2.5 kHz&lt;f.sub.d &lt;12.5 kHz; when Att=7.multidot.(f.sub.d -2.5 kHz) dB; and PA1 f.sub.d &gt;12.5 kHz; when Att=50+10log.sub.10 (RFOP) dB or 70 dB (whichever is smaller) PA1 f.sub.d =magnitude (in kHz) of a difference between an operating frequency and an emission component frequency; PA1 Att=required attenuation level (in dB), such that a power level of the emission component is below an unmodulated output power level for the data transmitter; and PA1 RFOP=RF Output Power (in Watts) for the data transmitter.
where;
That is, a transmitter's peak power curve 232 must fall below the mask 240 at all frequencies for legal operation of the transmitter, e.g., in an APCO-25 application. It should be noted that frequencies 234 and 236 generally define the center frequencies of the adjacent channels.
FIG. 2C shows a frequency domain transmitter power spectrum characteristic 260 that results for a different type of prior art constant envelope modulation, e.g., Motorola's so-called "Astro Wide Pulse Modulation" scheme, which was designed for high immunity to differential delay using a constant envelope transmitter. Note that while this modulation scheme is designed for the same bit rate as APCO-25 systems, and will pass the information in the APCO-25 protocol, it is not detectable by a standard APCO-25 receiver. In addition, this modulation's peak power density is shown to be outside the mask requirements 240 for a 12.5 kHz channel in the sideband areas 262, 264. Consequently, two channels must be used for simulcast operation, thereby reducing spectrum efficiency.
Given the foregoing limitations, it is important to note that an APCO-25 modulation system has the unique characteristic that allows receiver compatibility with a second type of transmitter modulation. As an example, FIG. 3 shows a simplified block diagram of a transmitter employing one such modulation scheme. This modulation scheme is designed to fit within a 6.25 kHz bandwidth, but is compatible with standard APCO-25 receiver operation. The improved bandwidth approach requires linear modulation of amplitude and phase, and linear amplification in the transmitter power amplifier. The information to be presently transmitted 102 is inputted to a multi-level encoder 304 followed by a differential encoder 306, as is known in the art The signal is then split into in-phase (I) and quadrature (Q) bit streams using the phase modulator 306. Both of these bit streams are passed through a processing function F(.omega.) designated by reference numbers 314,316. It should be noted that these processing functions are conventionally used to band limit or low-pass filter the modulated signal to control the transmitter power spectrum in a similar manner to the low-pass filter 104 shown in FIG. 1. These filters are conventionally designed to have impulse frequency response characteristics with square (very high corner slope) band pass traits, thereby minimizing intersymbol interference.
Mixers 318 and 320 are used to translate the base-band modulation signals to an RF center frequency, f.sub.C. The in-phase and quadrature signals are combined using a summer 322 to form a single RF signal 324. The modulated waveform 324 is then amplified using a linear amplifier stage 328 before being transmitted over the air, via a conventional antenna 110.
FIG. 4A illustrates the frequency domain power spectrum characteristics 430 that results from the transmission scheme shown in FIG. 3. The power density peak envelope curve 432 of this modulation scheme falls well within the required 12.5 kHz regulatory mask 240. Further, the peak power curve points 434, 436 are approximately 70 dB below the carrier power at approximately 6.25 kHz from the carrier frequency. As those skilled in the art will recognize, this modulation scheme is designed to minimize the bandwidth of the power spectrum. However, because of the uniform rate of phase change produced by the filtering functions 314, 316 shown in FIG. 3, there is no significant improvement in the multipath delay immunity characteristics using this modulation approach.
FIG. 4B shows an I and Q constellation diagram 400 that results from the amplitude and phase controlled transmission scheme known as CQPSK as employed by the transmitter of FIG. 3. Constellation points 403-407 constitute a plurality of the eight points defining modulation data values. The curve 401 (i.e., darkened trace) shows a transition through constellation points 403-404-405-406-407. Unlike the embodiments discussed earlier, phasors (409, 411, 413, 415) have different amplitudes throughout the phase transitions, but exhibit substantially constant angular velocity. By way of example, phasors 409, 411 illustrate a phase transition .0.1 requiring a duration of .DELTA.T 210, with slightly changing phasor amplitudes. Similarly, phasors 413, 415 show a phase transition .0.2 requiring the same duration .DELTA.T 210 , where .0.1 and .0.2 are substantially equal. That is, the phase angle change rates are approximately the same, whether the modulation is near a constellation point (e.g., phasors 413, 415) or midway between constellation points (e.g., phasors 409, 411). Of course, the relatively constant phase angle change rates on the transition curves are designed for smooth changes in phase, which indicates a tendency to produce a narrow bandwidth transmitter power density spectrum. Further, this modulation is designed to be compatible with the same receiver designed for the constant amplitude constellation shown in FIG. 2A, only having a higher level of spectrum efficiency.
Accordingly, there exists a need for a data transmission system that can be made compatible with an APCO-25 communications environment. Moreover, such a system should be compatible with today's standard APCO-25 receivers, and should not be constrained by the shortcomings of the prior art. In particular, a transmitter data modulation that meets the requirements of a standard APCO-25 receiver, while maintaining compatibility with a simulcast environment, would be an improvement over the prior art.